1. The Field of the Invention
The present invention relates generally to endodontic file instruments and methods for their manufacture. More specifically, the present invention relates to precipitation hardenable stainless steel endodontic instruments and methods for their manufacture that preferably do not include heat treatment.
2. Relevant Technology
To preserve a tooth with a pulp that is diseased or is potentially diseased, it is generally necessary to remove as much of the pulp material as is possible from the pulp canal of the tooth, to shape the root canal(s) without excessively weakening the root canal walls, to prevent or minimize the presence of bacteria through the use of irrigants and dressings, and lastly, to clean the walls of the root canal(s) by removing the smear layer created during instrumentation of the root canal(s). These steps are all done to prepare the root cavity for sealing or obturation which involves filling the root canal with biocompatible materials, such as gutta percha, before the pulp cavity is sealed, thereby promoting the healing and functional recovery of the tooth. This procedure is referred to as root canal therapy.
As indicated hereinabove, root canal preparation involves pulp removal, cleaning of the root canal walls and shaping of the canal walls. This is typically achieved through a guided procedure with the use of instruments which are moved either manually, mechanically or by combinations thereof These instruments are files or bits that are configured to bore and/or cut. Mechanical instrumentation can be achieved through the use of endodontic handpieces coupled to instruments such as files. The endodontic handpieces can impart rotational motion to a file, reciprocal motion by alternately rotating a file clockwise and counterclockwise, sonic movements or ultrasonic movements.
With regard to operating procedures, there are two basic methods from which all of the canal-preparation techniques can be derived. These methods have been interpreted by various authors in an operational context and also in terms of the instrumentation. The primary conventional systems and methods for removing pulp material from the root canal of a tooth are the apico-coronal (step-back) technique and the corono-apical (crown-down) technique. Although these conventional cleaning techniques both rely generally on sequential increases in the diameter of instruments inserted into the root canal. The step-back technique involves the sequential use of instruments by first inserting an instrument all the way down to the apex of the root canal and then using progressively larger and shorter files to clean the root canal. So the step-back technique involves cleaning the root canal from the apex toward the crown. The crown-down technique uses a set of files that are increasingly longer and smaller in diameter. So the crown-down technique involves first using a short and large diameter file to clean the upper portion of the root canal at the crown and then using a file that is longer and smaller until a file is used that reaches and cleans the apex. Each technique has its own unique benefits and disadvantages which are discussed hereinbelow.
As indicated above, in the step-back technique, the apical portion of the tooth is prepared first and then the remainder of the canal is flared from apex to crown. This process essentially involves inserting a series of progressively larger files into the apex of the root canal and rotating each file and/or moving the file up and down in a longitudinal.motion until a file can be inserted that is considered to be a suitable standard size for completing the process or that meets some resistance to rotation. The rest of the canal is then flared by sequentially using each file in the set with each file being larger than the preceding file and by alternately advancing and then withdrawing each instrument.
With each increase in file diameter, the rigidity increases and the flexibility of the files decreases. As a result, it becomes increasingly difficult for the files to adjust to or to follow the contours of the perimeter surfaces of the root canal. This reduced flexibility also increases the likelihood that the files will fail to contact some portions while removing too much of the surrounding dentin in some areas through excessive abrasion and resulting in overthinning of the walls.
Not only is the completeness effected by the use of a set of files wherein each file is more rigid than the preceding file but the ability to safely move the file within the canal is also limited. More particularly, the increasing rigidity results in decreased ability to negotiate the curves in the canal. Significant problems that can result from inserting increasingly rigid files and also from initially inserting a file all the way down to the apex includes laceration and transportation of the apical foramen, as well as misdirection and perforation of the wall.
Another problem is the formation of ledges. Ledges can occur when a practitioner attempts to insert a file as far as the apex and the file is too inflexible to properly curve with the root canal or move around a protrusion. When a file is too inflexible to curve or flex as needed and is halted prematurely, the downward pressure exerted on the file, in conjunction with the tendency of the file to straighten itself, causes the tip of the file to dig into the side of the root canal and form a ledge. Such ledges are difficult to bypass; and if the ledge occurs very close to the apex, the ledge may give the practitioner the mistaken impression that the apex has been reached.
The crown-down technique generally involves the use of a set of file instruments wherein each file in the set of file instruments has a progressively different diameter at the top of the cutting portion of the file, i.e., the point where the file becomes smooth and no longer has cutting capabilities. The smooth portion may have a constant diameter. The diameter at the top of the cutting portion of each file may be either constant or graduated for the entire set of instruments such that the top of the cutting portion of each file is progressively smaller than that of the preceding file. By using such files, a very large area is initially abraded and a large borehole is formed in the root canal.
One example of the operational deficiency of the crown-down method lies in its association with instruments made of nickel/titanium (Ni/Ti). Based on the greater flexibility of files formed from nickel/titanium compared with files formed from steel, proponents of the crown-down method in conjunction with nickel/titanium files assert that such files can better follow the curvatures of a root canal. Additionally, it has been asserted that such files are more likely to stay in the center of the root canal, thereby decreasing the likelihood of ledging or perforating the root canal walls. As set forth hereinbelow in greater detail, each material has its own unique advantages and disadvantages.
Moreover, because nickel/titanium files are more flexible than steel files, they tend to follow the path of least resistance and therefore cannot be used, in the same way as steel files, to be applied actively and intentionally by the operator. As a result, even when the operator knows the thickness of a particular portion, such as an interference or obstruction which the operator desires to rectify or straighten, the operator lacks the freedom to aggressively drive the file as needed and clean the portions that are difficult to reach. Accordingly, when a nickel/titanium file is used to clean a non-cylindrically shaped root canal, the file moves only at the center of the canal and/or the area of least resistance and fails to remove all of the necrotic tissue.
Problems, such as overthinning of root canal walls, perforation of a root canal wall, excessively weakening of the walls of the tooth and a failure to fully contact all of the canal walls, can be easily caused by the passive, self-guiding use of nickel/titanium files with progressively larger tapers in the transition from the first instrument to the next one in the set, and increasing rigidity in accordance with the crown-down technique which prevents the files from being laterally moved to enable the files to clean the entire perimeter of the root canal.
It should also be remembered that while rotation of a set of passively actuated files, in the center of the canal, in accordance with the crown-down technique, may result in successful root canal therapy, there is a significant hazard due to the passivity of the instruments when linked to canal diameters and wall thicknesses that are still statistically unknown.
The flexibility of the files used in the crown-down technique, which are typically formed from nickel/titanium, prevents the files from being successfully urged against the perimeter or against the various surface features of the root canal. As also discussed above, the flexibility of the files also increases the tendency of the files to remain in the center or at the location where less resistance to movement is encountered. Accordingly, the flexibility of the files also contributes to a borehole configuration which substantially deviates from the original anatomy of the root canal.
There are also other disadvantages to the use of nickel/titanium files. The flexibility of nickel/titanium files increases the likelihood that the file may bend and be deformed upon encountering a hard substance. Since nickel/titanium files are more fragile and more flexible than stainless steel files, the nickel/titanium files can break more easily and unexpectedly than steel files. When a nickel/titanium file instrument is used with a large file diameter the flexibility decreases to the point of being as rigid as stainless steel and yet breaks more easily. More particularly, beyond a certain diameter, the upper halves of larger diameter files are still as rigid as that of steel files while the flexible lower halves of nickel/titanium file instruments are more prone to break.
Additionally, rotation of a file in a canal that has a laminar upper two-thirds exposes the tip of the file to the risk of breaking when the tip of the file is embedded or stuck in a canal whose diameter is smaller than its own diameter! To avoid breaking the tip when it is embedded or stuck in a canal whose diameter is smaller than the diameter of the tip, operators who use nickel/titanium files are advised to employ catheterization in order to obtain a prophylactic widening of the canal, using a series of instruments with increasingly larger tip diameters.
Another disadvantage of nickel/titanium files is that nickel embodied in the alloy may potentially result in an allergic reaction. Further, nickel/titanium files cost about four times as much as steel files and yet nickel/titanium files generally wear out faster than steel files. Nickel/titanium files wear out so quickly that some manufacturers mark their products as being intended for single use only.
Although stainless steel of various types has been used in the manufacture of endodontic files, this material has been generically disqualified as being unsuited for its use as a single material in the manufacture of endodontic files. Reasons for this disqualification of stainless steel include its lack of flexibility and its stiffness that can lead to undesirable edging.
A conventional form of stainless steel in the manufacture of endodontic files is known as 304 stainless steel in the USA and as 18-8 stainless steel in Europe. This form of stainless steel, which is similar to USA 316 stainless steel, has generally the shortcomings that have been described above regarding stainless steel endodontic file instruments. To avoid the shortcomings that are conventionally associated with stainless steel endodontic files, the trend in endodontic has been to use other materials such as Ni/Ti alloys, superelastic Ni/Ti alloys, and two-material substances. See, for example, U.S. Pat. Nos. 5,213,499, 5,380,200, and 5,984,679, which are hereby incorporated herein by reference in their entirety.
As revealed by the present state of the art, stainless steel endodontic files have sufficient hardness, but they are not flexible enough for effective endodontic work. Ni/Ti endodontic files are more flexible than stainless steel files, but their flexibility is excessive for certain applications, they are brittle, and have low tensile strength, which easily leads to file rupture upon application of a torque. Consequently, Ni/Ti endodontic files are regarded as inadequate for use with variable torque handpieces such as air-driven turbines, even though such turbines have many desirable features for driving dental instruments.
Because some materials have the desired hardness and other materials have the desired flexibility, endodontic files have been manufactured with more than one material. However, multi-material file manufacture tends to be more complex than single material file manufacture.
Based on the foregoing observations, methods and devices are needed in the endodontic arts which enable a dental practitioner to effectively use file instruments which are more resistant to wear, harder and less flexible than Ni/Ti instruments, but not as hard and resilient as stainless steel instruments.
It would also be an advancement in the endodontic arts to provide methods and devices which rely on materials that are flexible enough and that have enough tensile strength without being unduly brittle, and without exhibiting the allergenic effects of Ni/Ti materials.
It would also be a beneficial development in the endodontic arts to provide methods and devices which can be used with variable torque handpieces, such as air-driven turbines, in addition to their use in hand-held instruments.
Additionally, it would be an advancement in the endodontic arts to provide methods and devices that can rely on only one manufacturing material.
Finally, it would also constitute progress in the endodontic arts to provide methods and devices in which the file material is sufficiently hard and such hardness is achieved without requiring heat treatment.
The present invention has been developed in response to the present state of the art and, in particular, in response to problems and needs that have not been solved heretofore.
An object of the present invention is to provide methods and devices that enable a dental practitioner to effectively use file instruments which are more resistant to wear, and which are harder and less flexible than Ni/Ti instruments, but not as hard and resilient as stainless steel instruments. An advantage of these features is that the devices according to the present invention are flexible while being stiff and hard enough to hold a sharp edge.
Another object of the present invention is to provide methods and devices that rely on materials which are flexible enough and which have enough tensile strength without being unduly brittle and without exhibiting the allergenic effects of Ni/Ti materials. An advantage of these features is that the devices according to the present invention are not so stiff as to easily cause ledging and are capable of withstanding stress without becoming plastically deformed or breaking.
An additional object of the present invention is to provide methods and devices that can be used with variable torque handpieces, such as air-driven turbines, in addition to their use in hand-held instruments. An advantage of this capability is that the devices according to the present invention are versatile and they can be effectively used in conjunction with an ubiquitous driver such as an air-driven turbine.
Additionally, another object of the present invention is to provide methods and devices that can rely on only one manufacturing material. An advantage of this manufacturing is that it reduces the complexity of the manufacturing process and it provides devices with the uniform properties of single material devices.
Finally, it is an object of the present invention to provide methods and devices in which the file material is sufficiently hard, and such hardness is achieved without requiring heat treatment. An advantage of this manufacturing feature is that it simplifies the manufacturing process because it requires fewer steps than conventional manufacturing processes that rely on heat treatment to impart the desired degree of hardness.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, file instruments according to the present invention comprise a file manufactured with a base material that is a precipitation hardenable stainless steel, preferably in the form of a type such as martensitic, semiaustenitic, and austenitic. The precipitation hardenable stainless steel according to the present invention is more preferably in the martensitic form, and most preferably a 17-4PH stainless steel.
Methods for manufacturing file instruments according to the present invention comprise the steps of providing a precipitation hardenable stainless steel base material and forming the material into a file. Files can be formed from the base material such as a precursor blank by an conventional method. Examples of conventional methods for forming a precursor blank or rod of base material into a file instrument include cutting lands into the rod, twisting the rod, abrading the rod, and combinations thereof Forming the file instrument may involve multiple steps such as imparting a desired longitudinal taper before cutting, twisting or abrading the precursor rod or blank used to form the file instrument. It may also be necessary to shape the precursor blank to provide a desired cross-sectional shape such as a triangular or square cross-sectional shape. The file instrument so obtained is preferably not subjected to heat treatment because it possesses the desired flexibility and tensile strength. When indentations or other features that are typically obtained by cutting are desired, such features can be cut into embodiments of the file instruments manufactured according to the present invention. The order in which cutting and twisting takes place is not a limiting feature of the methods of the present invention. This order is chosen in practice according to conventional machining preferences.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description, drawings, and appended claims, or may be learned by the practice of the invention as set forth hereinafter.